In recent years, the wireless technologies for video transmission have been widely studied, and a standard using millimeter-wave signals in the 60 GHz band is now being implemented. Although an active circuit operable in the 60 GHz band has been implemented as one chip by using the CMOS technologies with fine processing, it is difficult to increase the circuit output power because of a low drive voltage for transistors in the circuit. In addition, diffraction propagation can not be expected for 60 GHz-band electromagnetic waves, and thus, it is necessary to ensure a line of sight between a transmitter and a receiver. In the case of indoor wireless video transmission, since it is assumed that a user may often block a communication path, developers are weighing whether to use the beam steering technique for ensuring a line of sight while avoiding the blocking of the communication path by using reflections on walls, ceiling and floor. The aforementioned problem of low output power characteristics of a CMOS device is overcome by means of the array antenna technique in which powers from multiple antenna elements are spatially combined. However, in the millimeter-wave band with a shorter wavelength, the propagation loss becomes serious as compared to that of the microwave band, and thus, it is necessary to steer the antenna beam while maintaining a very high antenna gain. That is, a video transmission module operable in the 60 GHz-band needs to maintain its designed gain in any communication direction even after being mounted on equipment.
On the other hand, regulations require that undesired electromagnetic wave components radiated from electronic equipment be suppressed to a certain level or less in order to avoid adverse influences on other neighboring equipment. Accordingly, it is a common practice to use a shield structure for a housing of the equipment so as to prevent undesired electromagnetic waves from leaking from the inside of the equipment. Hence, in the case that a wireless communication interface is provided within the housing of the electronic equipment, then, conventionally, a transmitting and receiving antenna is mounted external to the equipment, or a part of the shield structure inside which the antenna is positioned is cut out so that an undesired electromagnetic radiation level satisfies the regulations.
Patent Literature 1 pertains to a radio radar apparatus that passes only radio waves with a certain frequency therethrough and suppresses the influence of external noise radio waves. The radar apparatus of Patent Literature 1 includes a shield structure made of a conductor and covering the radar's main body. The shield structure has a frequency-selective screen portion formed at at least its portion in front of an antenna. The screen portion is divided into a plurality of apertures by additional conductors whose both ends are short-circuited to an outer edge of the screen portion, and thus, the screen portion can block noise electromagnetic waves with frequencies fL lower than a certain frequency fp. In addition, by setting the resonance frequency of each aperture to the frequency fp, the screen portion can pass electromagnetic waves with the frequency fp therethrough.
Patent Literature 2 pertains to an electromagnetic shield structure that passes only a specific electromagnetic wave therethrough and blocks all the other electromagnetic waves. Patent Literature 2 discloses the electromagnetic shield structure including a conductive layer provided with a plurality of apertures. It is possible to pass only electromagnetic waves with a certain frequency therethrough by adjusting each aperture's length, and thus, when combined with the shielding by the conductive layer, obtain the same effects as those obtained by the invention of Patent Literature 1. The smaller each aperture's width w, the better the performance, and the allowable range of the width w is about 5% or less of each aperture's length 1. Patent Literature 2 reads, “if each aperture's width w is too large, the aperture's characteristics as slot antennas degrade”, and according to this statement, it can be seen that each aperture is operated as a slot antenna. Although Patent Literature 2 discloses a configuration for handling both of horizontal polarization and vertical polarization, the basic principles are the same as those of the invention of Patent Literature 1.
Patent Literature 3 pertains to a shield structure that passes radio waves in a specific frequency band therethrough and provides high blocking characteristics in the other bands. The invention of Patent Literature 3 is also configured in a manner similar to those of the inventions of Patent Literatures 1 and 2, and the invention of Patent Literature 3 adjusts a plurality of apertures divided from a screen portion so that each aperture operates as a slot antenna at a certain frequency, and thus, electromagnetic waves with that frequency can pass through the apertures resonating in that frequency. The invention of Patent Literature 3 differs from the inventions of Patent Literatures 1 and 2, in that, the invention of Patent Literature 3 is further provided with band-pass filters that each short-circuit between two opposing portions of each aperture's outer edge at frequencies fH higher than a certain frequency fp. Accordingly, the invention of Patent Literature 3 not only can pass electromagnetic waves with the frequency fp and block electromagnetic waves with frequencies fL lower than the frequency fp, but also can block electromagnetic waves with the high frequencies fH higher than the frequency fp.